Cable television distribution networks, or CATV distribution networks, have historically been used to provide a plurality of television signals from a centralized transmitter to a distributed network of subscribers. Such use typically required only one-way communication, from the centralized transmitter to the subscribers. Recently, however CATV distribution networks have been increasingly employed for two-way communication to facilitate the provision of new services. CATV distribution networks are desirable for use as two-way communication links because of the bandwidth that is available in the distribution network. CATV distribution networks can support high speed data links that may be used for computer internetworking, home banking, and even telephony.
All communications, including communications over CATV networks, require a high quality transmission network. Accordingly, CATV service providers closely monitor the operation of the CATV distribution network to ensure that subscribers receive appropriate signal levels. To ensure quality two-way communications CATV service providers also perform tests to monitor the quality of reverse path transmissions, in other words, transmissions from the subscribers to a centralized receiver. Unfortunately, most of the problems associated with reverse path transmissions originate within devices and components owned and maintained by CATV subscribers.
In particular, CATV subscribers typically own and maintain the distribution network, or subscriber network, located within their own dwellings. Accordingly, the component quality and condition can vary widely. Moreover, CATV subscribers often install one-way amplifiers and other components that are not intended for two-way communications. While such components may provide adequate signal levels for receiving television signals, such components are often inadequate for two-way communication applications.
Accordingly, with the advent of two-way communications using the CATV distribution network, a need has arisen for testing the signal response of subscriber networks to ensure high quality reverse path transmissions. One effective method of testing the signal response of a network such as a subscriber network is a test known as a frequency sweep test, or simply sweep tesi. A sweep test is a test in which a transmitter is connected to a first end of a system under test and transmits a signal having a swept frequency over a predetermined frequency range. A receiver that is synchronized with the transmitter then is connected at a second end of the system under test and receives the signal and analyzes the received signal strength at each of the swept frequencies. The analysis provides the frequency response of the system under test.
CATV service providers have historically used sweep testing to test the forward path signal quality of the CATV distribution network. Several sweep testing systems are known. These systems, such as the one disclosed in U.S. Pat. No. 5,585,842, are primarily intended for a configuration in which the sweep transmitter is installed at the CATV centralized transmitter, and the sweep receiver is installed at a remote test site. Typically, the sweep receiver will be moved from test site to test site while the transmitter is installed at the CATV centralized transmitter.
One consequence of the above described configuration is that the sweep transmitter must communicate with the sweep receiver to coordinate sweep plan information and normalization measurement information. Sweep plan information is information that identifies the frequencies to be swept, which can vary from test to test. Normalization measurement information is information identifying the strength of the transmitted sweep signal, which can vary from test to test, particularly over long periods between tests.
U.S. Pat. No. 5,585,842 teaches the communication of the sweep plan information and normalization measurement information by transmitting a telemetry signal over the CATV network to the sweep receiver. The telemetry signal is a baseband data signal modulated onto an RF carrier signal. The baseband data signal comprises data representative of the sweep plan and the normalization measurements. The sweep receiver then uses the sweep frequency information in the telemetry signal to identify the frequencies to be swept, and uses the measurement information to identify the strength of the transmitted signal. To transmit telemetry signals over the CATV network, the sweep transmitter includes circuitry for modulating a digital information signal onto an RF carrier signal.
Sweep systems of such design are quite adequate for use in configurations in which the sweep transmitter is installed at the head end and the sweep receiver is moved from location to location, as is typical in forward path measurements. Sweep systems of such design have also been used for testing subscriber networks in the context of reverse path communications. However, such systems are not cost optimal for testing reverse path communications of subscriber networks. In particular, the telemetry signal transmission capabilities significantly impact the cost of the sweep transmission devices used in such systems. While such costs are easily justified for forward path testing, in which only one sweep transmitter is needed to test an entire network, such costs are not always justified for reverse path testing of subscriber networks, where a separate transmitter is required for each test.
In particular, to test a subscriber network in isolation, the sweep transmitter and sweep receiver must be installed at each test location. Accordingly, in contrast to forward path distribution network testing, a separate transmitter is required for each testing operation. To carry out several tests in parallel, or to allow several technicians to have the necessary equipment to carry out such tests, the CATV service provider must stock several sweep transmitters. Such an increase in the number of transmitters owned by a CATV service provider greatly increases the cost to the CATV service provider. Thus, for reverse path subscriber network sweep testing, a need has arisen for a low cost and low complexity sweep measurement system.
One way of reducing the sweep transmitter cost would be to use a single predefined sweep plan and dispense with normalization measurements, thereby eliminating the need for telemetry signals. Telemetry is necessary, however, to facilitate flexible and accurate tests. In particular, the normalization measurement information provided through telemetry signals is required for accurate signal response measurements. Without the normalization measurement information, which provides a measurement of the transmitted signals at the transmission point, operational variances of the sweep transmitter are not accounted for in the measurement thereby causing inaccuracy. In addition, without the sweep plan information, the sweep transmitter and sweep receiver must rely on a predetermined and inflexible set of frequencies to be swept.
A need therefore exists for a sweep testing system that has the accuracy provided by the use normalization measurement information identifying the strength of the sweep signal at the transmission point, or normalization information, without the need for providing RF telemetry signals. A need also exists for a sweep testing system that has the flexibility provided by a variable sweep plan without the need for providing RF telemetry signals.